This invention pertains to liquid epoxy molding systems and particularly to the addition of a mixture of organic carboxylic acid and silica to the system to thicken it prior to cure.
The use of liquid epoxy molding compounds for transfer and injection molding applications are relatively new. In comparison with solid epoxy molding materials, which are extensively used for the encapsulation of electronic components and the manufacture of electrical insulators, the liquid epoxy resins offer the ability to be molded under low pressures, that is, about 5 psi to about 1000 psi, with the result that large moldings can be processed in presses with relatively low tonnage clamping capacities. This also makes possible the use of such liquid epoxy resins as liquid cycloaliphatic epoxy resins for normal transfer molding operations.
The use of liquid epoxy molding compounds for transfer and injection molding applications, however, is hampered by two serious problems. Firstly, the low viscosity of the liquid epoxy resin contributes to poor rheological properties during the injection into the mold with the result that voids or air inclusions become a problem. Secondly the low viscosity of the system also contributes to resin seepage through the land area of the mold as well as to excessive wetting of the mold cavity. The latter effect interferes with the release of the molded article from the mold cavity. The former effect makes it difficult to remove resin flashing which creates a problem serious enough to render the process of molding liquid epoxy resins impractical.
Attempts to solve the resin seepage problem by incorporating solid cycloaliphatic epoxy resins with liquid epoxy resin in the molding composition lessened void formation but did not provide a satisfactory commercial process.
Another approach to the solution of the problems associated with molding liquid epoxy resins involved modifying the normal operation of a reciprocating screw injection machine. The modification consisted of intentionally advancing the molding system at an elevated temperature and specified dwell time so that the viscosity of the molding composition injected into the mold was many times higher than would be the case if processed according to accepted practice for injection molding. Marginal improvements were effected which did not give completely acceptable results.